Which ligand binds the tightest? ligand a, with a kd of 10^-9m ligand b, with a kd of 10^-3m ligand c, with a percent occupancy of 30% at one micromolar
ligand d, with a percent occupancy of 80% at 10 nm

To answer this item, we must take note that the ligand that binds the tightest is the one with the lowest dissociation constant, Kd. Kd's for both A and B are already given so, we only need to solve Kds for C and D. 
  Kd of C
                        0.3 = (1x10⁻⁶)/(1x10⁻⁶ + Kd)   ; Kd = 2.3x10⁻⁶
 Kd of D  
                        0.8 = (1x10⁻⁹)/(1x10⁻⁹ + Kd)   ; Kd = 2.5x10⁻10
Since Ligand D has the least value of dissociation constant then, it can be concluded that it binds the tightest. 

Qrepresents joules in the formula